A. Technical Field
The present invention relates to common mode controls and to systems, devices, and methods of controlling common mode voltage in capacitive inertial sensor circuits.
B. Background of the Invention
Capacitive sensing circuits are used in gyroscopes, pressure sensors, accelerometers, etc. to sense a change in capacitance value caused by a linear or rotational movement. A differential change can be detected by a differential operational amplifier that outputs a proportional voltage, which then can be converted into the desired physical quantity to be detected, for example, rotation, pressure, or acceleration.
A capacitive sensing circuit operates through a driving signal, like a voltage step, provided to a capacitive sensor and the front end amplifier that reads and amplifies the sensor signal. Typically, a fully differential input charge amplifier is used since fully differential amplifiers are more reliable, accurate, and relatively immune to noise generated by the power supply. However, fully differential charge amplifiers generally require control of the input common mode voltage through a dedicated circuit, which can have a significant negative impact on the amplifier's performance. The fully differential front end charge amplifier and its common mode voltage control for the capacitive inertial sensor are often implemented using switch-capacitor circuits.
Switch-capacitor input common mode voltage control circuits require a relatively a large feedback capacitance that increases the total input front end capacitance and thereby increases system noise and decreases the amplifier's performance. In addition, the inability of the common mode feedback amplifier to quickly and precisely recover the desired common mode voltage reduces the maximum available operation speed of the front end electronics. What is needed are common mode voltage controls that overcome the above-described limitations.